1. Field of the Invention
This invention relates to a method for producing multitone image data from data representative of an original image and, in particular, for producing multitone image data with a set of N output darkness levels U greater than 2 from original darkness values V.sub.o representative of an original image. An alternative approach is described in a commonly assigned application to Michael Speciner, Ser. No. 07/810,462, now allowed.
2. Description of Related Art
Any photograph, painting, etc., consisting of a broad range of tones or gradation of tones is known as a continuous tone image or picture. In paintings, changing the pigment concentration or the amount of deposit of the printing media on a substrate modulates the color values throughout an entire picture. In photography, the different tones are created by varying amounts of silver, which appears black, in image areas.
In most forms of printing, tones cannot be reproduced by varying concentrations or amounts of ink. Many printing processes can print only a solid density of a color in an image area, while no ink prints in a non image area. In order to reproduce the varying tones of a continuous tone original image, a halftone process can be used to convert the original image into a pattern of very small and clearly defined dots. More specifically, the original image can be scanned with a light beam using an image scanner to obtain an electronic signal containing image information in the form of intensity levels or optical density variations corresponding to elemental picture areas, known as pixels, of the original picture. Then a thresholding technique can be used to generate data or an output that can cause a dot to be printed in a pixel of the halftone reproduction if the intensity level or optical density variation of the corresponding pixel in the original image exceeds a predetermined value. The output can be stored in a memory or used directly to drive a recorder, such as an exposing device, a printer or a computer display.
In halftoning processes, each dot in the reproduced image has the same tonal value or optical density. Various tones in the original are represented in the halftone image by a large number of evenly spaced dots which can have different sizes. Larger dots produce darker tones. Smaller dots or fewer dots make lighter tones. Halftones have the appearance of continuous tone images because of the limited resolving power of the human eye. When viewed at a normal reading distance, the individual dots blend into and appear like a "continuous" tone.
A rather different halftoning technique is based on the idea that there is an error associated with each halftone pixel which is the difference between its darkness value (which can only be reproduced by a printed dot or the absence of a printed dot) and the darkness value of the corresponding continuous tone original image pixel. If this error is distributed to neighboring pixels before the halftone thresholding is performed for them, the sum of the errors in a region can be minimized, thus faithfully reproducing the average darkness value in the region. The Floyd-Steinberg algorithm produces good halftone images using this technique. For further information on the Floyd-Steinberg algorithm, see "An Adaptive Algorithm for Spatial Grey Scale," by R. W. Floyd and L. Steinberg in SID 75 Digest, Society for Information Display, 1975, pages 36-37.
In the reproduction of color images, the original multicolored image is typically scanned through separate sets of colored filters to produce four separate image signals, corresponding to three primary colors and, possibly, black. The chosen primary colors for color printing are typically cyan, magenta and yellow. The chosen primary colors for a color computer display are typically red, green and blue. The separate signals are processed as described above. In other words, a color separation signal representative of a pattern of very small and clearly defined dots is generated for each of the colors where the dots in each of the patterns has the same tonal value or optical density throughout that pattern. Then the patterns for each color must be combined.
For instance, an exposing device may produce black and white film transparencies, corresponding to each of the cyan, magenta, yellow and black color separation signals, known as color separation transparencies or color separations for short. The color separations are used to make printing plates. The printing plates are mounted on a printing press to print, in registration, four overlapping single colored images to reconstruct the multicolored original.
The electronic output signals may also be used to directly drive a color printer or a computer display to produce a reconstructed multicolored image without ever producing actual color separations or printing plates.
It is desirable to provide a method for producing a multitone image using a set of at least 3 output darkness values (or levels) for at least one output color from data representative of an original image.